If you have installed the mongodb cartridge like the instructions, before deploying change this line in meteorshim
process.env.MONGO_URL = process.env.MONGODB_URL || "PLEASE_PROVIDE_A_MONGO_URL";
with the proper address of a database connection uri. By default the
process.env.MONGODB_URL has no database or users so you should create one to connect.
In order to create a new database, do
rhc port-forward -a yourappName -n yourNamespace(not needed if only one namespace)
you will see something like
Service Local OpenShift
mongod 127.0.0.1:27017 => xxx.yyy.zzz.www:ppp
Then you can connect to the remote database in robomongo or any other mongodb management gui tool (choose your favourite from here)with the ip 127.0.0.1 and the port 27017 in this example.
On robomongo or whatever create a new database with the name that you want (meteor for example) and then look also in the gui to create a new user on that database. Once you have both things done then you can paste
process.env.MONGO_URL = "mongodb://username:email@example.com:ppp/databasename" in meteorshim.js
As an alternative way If you can't manage, go and create a free database at mLab repeat the same steps of creating a database and a user/password (it's easier since you can do this directly from their website) and then look for this text in your webpage( after clicking on the database)
To connect using a driver via the standard MongoDB URI (what's this?):
copy that url to meteorshim replacing the dbuser, dbpassword, xxxxx, ppppp, database name values and voila!